The exhaust gas from an internal combustion engine such as automobile engine contains nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbon (HC) and the like. Therefore, such an exhaust gas is generally released into the atmosphere after the purification with an exhaust gas purifying catalyst capable of oxidizing CO and HC and, at the same time, reducing NOx. As for the exhaust gas purifying catalyst, a catalyst obtained by loading a noble metal such as platinum (Pt), rhodium (Rh) or palladium (Pd) on a porous support such as γ-alumina is generally used.
Also, in recent years, it is known to obtain the exhaust gas purifying catalyst by loading, in combination with the noble metal, an alkali metal and/or an alkaline earth metal on the support. As for such a catalyst using an alkali metal and an alkaline earth metal, there is known an NOx storage-reduction catalyst that stores NOx when the air-fuel ratio is in a lean state, and reduces the stored NOx into N2 when the air-fuel ratio enters a rich state, for example when a rich spike is performed at regular intervals. This NOx storage-reduction catalyst is a catalyst where a noble metal such as platinum, rhodium and palladium and an NOx storage material selected from alkali metal and alkaline earth metal elements such as potassium and barium are loaded on a porous oxide support such as alumina.
In the conventional production of the NOx storage-reduction catalyst, generally, an alumina support coated on a substrate is dipped in a noble metal solution such as aqueous dinitrodiamine platinum nitrate solution, dried and fired, and the support on which a noble metal is thus loaded is dipped in an alkali metal or alkaline earth metal salt solution such as aqueous barium acetate solution, dried and fired to produce the catalyst (see, Japanese Patent No. 3,311,012).
However, it is known that when an exhaust gas purifying catalyst is produced in this way by using a metal salt, particularly a barium salt, as described in Japanese Unexamined Patent Publication No. 2000-279824, such an alkaline earth metal particle exhibits insufficient dispersibility in the obtained exhaust gas purifying catalyst. In this regard, in the prior invention concerned, sintering of platinum is suppressed by using a composite metal colloid such as platinum-rhodium, and barium is loaded to high dispersion in the vicinity of platinum, whereby an automotive exhaust gas purifying catalyst having excellent NOx storage ability is provided.
Incidentally, as regards high dispersion loading of a catalyst component on a support, in Japanese Unexamined Patent Publication No. 11-192432, a noble metal cluster carbonyl compound is used, and thereby a noble metal is loaded to high dispersion on a support by utilizing large affinity between the compound and the hydroxyl group on the support surface.